1. Field of the Invention
The present invention relates to a developing roller, a developing device and an image-forming apparatus.
2. Description of the Related Art
Conventionally, in an electrophotographic printer, the surface of a photosensitive drum is evenly charged by a charging roller. Next, an electrostatic latent image is formed on the surface of the photosensitive drum by an exposing device, and toner is adhered to the electrostatic latent image by a developing device, so that the electrostatic latent image is visualized to form a toner image. Moreover, the toner image is transferred onto a medium by a transferring roller, and fixed thereon by a fixing device.
In the developing device, a mono-component developer containing only the toner is preferably used from the viewpoint of miniaturization of the device. In the developing device of this kind, a developing roller, which transports the toner to a developing area, is made in contact with, or brought close to the photosensitive drum so as to freely rotate thereon. Here, a supply roller that supplies the toner to the developing roller is made in contact with the developing roller so as to freely rotate thereon. Moreover, a blade that regulates the toner on the developing roller is placed with its tip being made in contact with the surface of the developing roller. In the developing device having this structure, the toner, housed in a toner cartridge, is supplied onto the developing roller by the supply roller, and the toner thus supplied is formed into a thin-film state by the blade in accordance with the rotation of the developing roller, and charged with a predetermined polarity to form a toner layer. The toner of the toner layer is transported to a developing portion between the developing roller and the photosensitive drum, and is electrostatically adhered onto the electrostatic latent image on the photosensitive drum, so that the electrostatic latent image is visualized to form a toner image.
With respect to the developing roller, in general, those having a resin layer on the surface are used, and organic or inorganic fine particles are dispersed in the resin layer so as to improve the toner transporting property.
For example, a developing sleeve, which has a coat layer on its surface, and has conductive inorganic fine particles contained in the coat layer, with fine concave and convex portions being formed on its surface, has been reported (Japanese Patent Application Laid-Open No. 8-160738). However, in the case when such a developing sleeve is used for a long time, the convex portions on the surface, formed by the conductive inorganic fine particles, are worn to cause the problem that the particles relatively easily come off from the surface. The separation of the particles causes a failure to provide sufficient toner charging property and transporting property. Moreover, the charging stability against environmental variations is lowered. In the case when the charging stability against environmental variations is lowered, since the change in the quantity of charge in the toner becomes extremely high due to variations in the ambient environment, the developing property is lowered, failing to provide sufficient image density to cause fogging and image irregularities. Therefore, a mechanism, such as a system to be used for measuring the ambient environmental factors (temperature, humidity and the like) so as to stabilize the developing property, is required, resulting in the problems of bulky devices and high costs due to an increase in the number of parts.
Here, for example, a developer-supporting member having a coat layer on its surface, with concave/convex portion-imparting particles indefinite in shape being contained in the coat layer, has been reported. However, such a developer-supporting member results in the problem that the particles are aggregated on the surface thereof. When the particles are aggregated, concave/convex irregularities occur on the surface to cause unevenness in the image density. Moreover, the problem of insufficient charging stability against environmental variations is also caused.